1. Field of the Invention
The present invention relates to signal storage devices in general and, in particular, to apparatus for use in interrogating individual cells of such devices.
2. Discussion Relative to the Art
It is well known to interrogate arrays having individual storage cells that influence a discharge path from a signal line by precharging the signal line and, in effect, detecting the rate at which the charge is drained. A particularly sensitive detection can be achieved by comparing the discharge for the signal line of the interrogated cell to a reference discharge; generally a discharge that occurs at a rate intermediate to the rates for the two possible states of an interrogated cell.
The cells may, for example, be a string of field effect transistor (FET) switches that form a series connection to ground from a signal line. All of the switches of the string, except the switch under interrogation, are forced to conduct and the gate electrode of the interrogated transistor cell is set to a level such that it will conduct if it is implanted to be a depletion mode device (one stored logic state) and will not conduct if implanted to be an enhancement mode device (the other logic state).
If plural signal lines are used, all lines are typically precharged and the particular signal line of the interrogated cell is switched to supply the state representative signal to a detector. And, if the above mentioned comparison detector is used, a reference signal is also supplied as an input to the detector.
Some problems of the usual configuration relate to the amount of space consumed for switching respective signal lines and the timing requirements for such switching. Also, manufacturing variations for the reference and cell circuits require a greater discharge to occur than is desirable before a reliable detection can be achieved.